Anode

35

36 desert basins created by evaporation of the water previously contained. The most abundant sources of lithium-rich brine are found in South America, particularly in Argentina, Bolivia and Chile, collectively known as the lithium triangle as it contains 50% of the world's lithium reserves.[39] In all these countries, the sources are located at high altitudes, where the atmospheric pressure is low, and the ability of the water to evaporate is high. Some studies show that essential quantities of lithium are also found in geothermal waters (due to the higher leaching capacity of hot waters compared to cold ones) and in oil fields brines. [24] Currently, lithium is not yet extracted from these sources, but they could constitute a valid alternative for the future.Lithium, instead, is mostly extracted as lithium carbonate or lithium chloride from saline desert basins thanks solar evaporation. Evaporation helps to concentrate lithium in a gradually smaller brine volume. After reaching a sufficient concentration of the brine, lithium is extracted by chemical means, as lithium chloride or, more commonly, lithium carbonate, through precipitation, adsorption, and / or ion exchange. The whole process can take 18 to 24 months. [39] [36]

Nowadays, it is estimated that around 25% of the world's lithium resources are found in minerals, 59%

in brines and the remainder in clays, geothermal waters and oil fields brines.

Initially, all lithium was extracted from hard rock mineral sources, but since the 80s, the discovery of mineral-rich brines such as those found in the Andes (Latin America), led to a change in trend, resulting in the closure of several lithium mines. However, there was never a definitive transition between these technologies as often the high lithium concentration in mineral sources can offset the high costs associated with the extraction processes.

For this reason, lithium present on the market today can be divided as follows (Figure 2.20 b):

➢ 50% from salt brines (dry saline lake beds)

➢ 40% deriving from the extraction of minerals from mines

➢ 10% from clay deposits and other sources [36]

Figure 2.20 : (a)Lithium distribution by country in 2016 and, (b) by source. [39]

Even though China needs to import a massive quantity of lithium, economically, it dominates the manufacturing industry of lithium products thanks to its ability to sell low-cost products. On the other hand, even if the European Union is one of the biggest consumers of lithium in the world, it manages to produce only a negligible amount of metal (1-2%).[39] [36]

The substantial discrepancy between these two world powers worries researchers. According to some studies, thanks to the discovery of new deposits, and advancement of extraction technology and

(a) (b)

37 recycling techniques, Europe will be able to meet its future demand. According to others, the lithium demand could exceed the extraction and production ability in the future not only at European level but worldwide.

Currently, lithium is not considered like a CRM (Critical Raw Material) as it is quite abundant on a global level and because on the market, there are other materials able to replace lithium in certain technologies (e.g. manganese and nickel in batteries). However, it is estimated that due to the uncertainties surrounding the supplying of this metal, lithium could become part of the CRM category shortly. [36]

Nowadays LIB (Lithium Ion Battery) production consumes the most significant percentage of lithium on the market (Figure 2.21 and 2.22), however not all LIBs require the same amount of lithium carbonate. This quantity depends on the applications and capacity required for the device. A smartphone battery, for example, needs about 3g of mineral, a PC battery a quantity varying between 10 and 30 g, a power tool battery of 40-60 g and a battery for electric vehicles of 8-100kg. [39] [36]

Figure 2.21: Diagram showing the annual consumption of Lithium and the industrial sector in which it is used [36]

Figure 2.22: Several pie charts (one in each year considered) which shows how Lithium consumption varies in the various industrial sectors over time [36]

38 According to a recent study a 60 kWh LIB in each car (currently 1 billion) would consume 50% of the world's lithium reserves. This scenario does not take into account goods vehicles that usually require batteries at least ten times larger than a generic electric vehicle. [36]

SEI formation

As previously seen, the intrinsic lithium properties make it an extremely reactive material, with both positive and negative consequences on the performance of the cells.

On the one hand, this characteristic allows it to significantly contribute to the high voltage of the cells, making lithium an optimal electrode material. On the other hand, its uncontrollable reactivity leads it to react also with most of the elements which come into contact, giving rise to unwanted reactions. These side reactions are at the basis of an irreversible consumption of lithium and a consequent capacity fade, early degradation of the cell or both. [40]

One of the most famous side reactions inside the cell is that involves the solvent, present in the electrolyte, and lithium anode: during the reaction the solvent is consumed causing an increase in the electrolyte viscosity and changes in the kinetics system. However, this reaction produces also an unexpectedly positive effect: in the contact area between lithium and electrolyte, an interface known as SEI (Solid-Electrolyte Interface) is formed. This passivation layer can protect lithium from further unwanted reactions, reducing degradation phenomena and therefore improving the cyclical life of the cell. [17]

The growth of the SEI proceeds if the transfer of electrons (electron tunnelling) is allowed. This phenomenon generally occurs up to a thickness of a few tens of angstrom. The rate at which the SEI layer grows must guarantee the layer homogeneity and its ability to regenerate at each cycle. A too-fast growth rate proves to be ineffective in order to create a uniform layer as well as a too slow reaction rate, does not allow the layer to regenerate in time for the next cycle, leaving areas of the anode uncovered.

The resulting layer can ideally be split into two sub-layers: a porous outermost film (in contact with the electrolyte) and a film in close contact with the lithium anode, with a compact structure. The outermost layer porosity is linked to the reduction of the species in solution, which does not occur homogeneously over the entire surface. Due to the nature of the process, reactions occur more easily near defective areas such as surface holes where electrons can tunnel to the surface. If the surface has many defective zones, the formation of dendrites can be induced.[41] Moving towards the lithium anode lower oxidation states are found which make the SEI more compact. The morphological variation occurs gradually within the layer.

The SEI layer is also the region where the first electrochemical reactions occur, for this reason, the nature of the layer can heavily influence the reaction kinetics: a disordered structure, for example, can favour ionic conductivity. However, if the structure is too rough during the charging and discharging phases, irregular plating and depleting phenomena can occur. This phenomenon generates an increase in lithium ions and a new SEI formation, with consequent consumption of lithium and electrolyte and early cell degradation. Also, the thickness of the SEI may have an impact on the performance of the cell.

An increase in thickness layer causes an increment in internal resistance and a slowdown in the kinetics of the electrochemical process. An ideal SEI should, therefore, allow lithium ions to pass but it should also be chemically stable, compact, uniform and have rigidity and elasticity such as to adapt to the volume changes that involve the material.[17] [29]

In order to obtain the best compromise between these properties, it is necessary to control the electrolyte and additives composition. In this scenario, lithium nitrate, an additive that is usually added to limit the shuttling effect, has also proven to be an excellent ally for the formation of a protective SEI layer.

Furthermore, unlike most oxides, sulphides and halides, which undergo reduction by the lithium metal, the nitride anion shows unique stability due to its intrinsic properties and through passivation characteristics. By contrast, lithium nitrate is depleted during regular cell operation due to the constant repair of the SEI, in which it is involved. For this reason, its effectiveness is reduced with time.

39 Moreover, although it is the most followed approach nowadays, the addition of additives negatively affects the energy density of the cell, internal resistance and cyclic stability. For this reason, researchers are also looking for other solutions that do not provide their use. [29] [31]

In conclusion, SEI is a fundamental element for the correct functioning of the cell, as its presence allows to reduce excessive consumption of the solvent and the problems that this entails. At the same time, if not correctly designed, the SEI can negatively influence the performance of the cell. For this reason, many research works are focus on the attempt to develop new solvents and reactive additives capable of forming resistant SEI or even to develop artificial SEI. [17]